High dynamic range gyroscope

ABSTRACT

A sensor includes an acceleration or magnetic field sensitive microelectromechanical systems (MEMS) resonator, configured to oscillate in at least a first normal mode and a second normal mode. The sensor further includes: a coarse readout circuit configured to drive the first normal mode, measure a motion of the first normal mode, and derive from the measured motion a coarse measurement of the true acceleration or true external magnetic field; and a fine readout circuit configured to drive the second normal mode, measure a motion of the second normal mode, and derive from the measured motion and the coarse measurement a measurement of the difference between the true acceleration or true external magnetic field and the coarse measurement.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a divisional of U.S. patent application Ser.No. 16/277,610, filed on Feb. 15, 2019, which is a divisional of U.S.patent application Ser. No. 15/253,694, filed on Aug. 31, 2016, whichclaims priority to and the benefit of U.S. Provisional Application No.62/212,902, filed Sep. 1, 2015, entitled “ATOM LOCKED ANGULAR SENSOR(ATLAS) WITH HIGH RESOLUTION AND DYNAMIC RANGE MULTI-MODE GYRO (MMG)ARCHITECTURE”, and priority to and the benefit of U.S. ProvisionalApplication No. 62/321,042, filed Apr. 11, 2016, entitled “STABILIZATIONOF CORIOLIS VIBRATORY GYROSCOPES BY FREQUENCY LOCKING TO ULTRA STABLECLOCKS”, the entire contents of all of which are incorporated herein byreference.

FIELD

One or more aspects of embodiments according to the present inventionrelate to angular sensors, and more particularly to an angular sensorwith high dynamic range.

BACKGROUND

Gyroscopes may be used in a wide range of applications, includingguidance of aircraft, spacecraft, missiles, and the like. A gyroscope(or “gyro”) measures an angular rate, i.e., the rate at which thegyroscope rotates, about one or more axes. The output of a gyroscope maybe a digital data stream. The rate resolution of the gyro, i.e., theability of the gyro to detect low angular rates or small changes inangular rate, may be limited in part by the resolution (i.e., the numberof bits) and scale factor of an analog to digital converter (ADC) thatmay be part of a signal chain connecting a physical sensing element to adigital output of the gyro. The range of the gyro, i.e., the maximumangular rate that it is capable of measuring, may also be related to theresolution and the scale factor of the ADC. As such, a gyro designed tooperate at high angular rates may have relatively poor resolution, and ahigh-resolution gyro may have relatively limited range. Someapplications, however, may require a gyro having both high range, e.g.,in aircraft or missiles designed to be highly maneuverable, and fineresolution, to provide accurate guidance.

Thus, there is a need for a gyro with high dynamic range, i.e., withhigh rate resolution and high range.

SUMMARY

According to an embodiment of the present invention there is provided anangular sensor, including: a Coriolis vibratory gyroscope (CVG)resonator, configured to oscillate in: a first pair of normal modesincluding a first normal mode and a second normal mode; and a secondpair of normal modes including a third normal mode and a fourth normalmode, a coarse readout circuit configured to: drive the first pair ofmodes, measure motion of the first pair of modes, and derive from themeasured motion of the first pair of modes a coarse measurement of atrue angular rate of the CVG resonator; and a fine readout circuitconfigured to: receive the coarse measurement, drive the second pair ofmodes, measure motion of the second pair of modes, and derive, from: themeasured motion of the second pair of modes, and the received coarsemeasurement, a measurement of the difference between the true angularrate of the CVG resonator and the coarse measurement.

In one embodiment, the fine readout circuit is configured to drive thethird normal mode at a first drive frequency and to drive fourth normalmode at a second drive frequency, and wherein the fine readout circuitis configured to derive the measurement of the difference between thetrue angular rate of the CVG resonator and the coarse measurement byadjusting the first drive frequency and the second drive frequency sothat the difference between the first drive frequency and the seconddrive frequency is proportional to the coarse measurement.

In one embodiment, the fine readout circuit is configured to derive themeasurement of the difference between the true angular rate of the CVGresonator and the coarse measurement by adjusting a natural frequency ofthe third normal mode and a natural frequency of fourth normal mode sothat the difference between the natural frequency of the third normalmode and the natural frequency of the fourth normal mode is proportionalto the coarse measurement.

In one embodiment, the fine readout circuit is configured to adjust thenatural frequency of the third normal mode by adjusting a bias voltageapplied to a tuning electrode coupled to the third normal mode.

In one embodiment, the fine readout circuit includes: a secondtransimpedance amplifier configured to measure a displacement in thefourth normal mode; an analog summing circuit connected to an output ofthe second transimpedance amplifier; a modulator block configured togenerate a waveform that is the opposite of the waveform expected at theoutput of the second transimpedance amplifier when the angular sensor isrotating at a rate indicated by the coarse measurement; and a digital toanalog converter connected to an output of the modulator block, andwherein the analog summing circuit is configured to add the output ofthe digital to analog converter to the output of the secondtransimpedance amplifier.

In one embodiment, the fine readout circuit includes: a secondtransimpedance amplifier configured to measure a displacement in thefourth normal mode; an analog to digital converter connected to anoutput of the second transimpedance amplifier; a digital summing circuitconnected to an output of the analog to digital converter; and amodulator block configured to generate a waveform that is the oppositeof the waveform expected at the output of the analog to digitalconverter when the angular sensor is rotating at a rate indicated by thecoarse measurement; and wherein the digital summing circuit isconfigured to add the output of the modulator block to the output of theanalog to digital converter.

In one embodiment, the fine readout circuit is configured to generatethe measurement of the difference between the true angular rate of theCVG resonator and the coarse measurement with a resolution of 19 bits.

In one embodiment, the coarse readout circuit is configured to generatethe measurement of the difference between the true angular rate of theCVG resonator and the coarse measurement with a resolution of 19 bits.

In one embodiment, the coarse readout circuit is configured to drive thefirst pair of modes so that an amplitude of motion of first normal modeis about 10 times an amplitude of motion of the second normal mode.

In one embodiment, the coarse readout circuit is configured to drive thefirst pair of modes so that a phase of motion of first normal mode isabout 90 degrees different from a phase of motion of the second normalmode.

In one embodiment, the fine readout circuit is configured to drive thesecond pair of modes so that an amplitude of motion of the third normalmode is about 10 times an amplitude of motion of the fourth normal mode.

In one embodiment, the fine readout circuit is configured to drive thesecond pair of modes so that a phase of motion of the third normal modeis about 90 degrees different from a phase of motion of the fourthnormal mode.

In one embodiment, each of the first pair of modes has a mode number of3 and each of the second pair of modes has a mode number of 2.

In one embodiment, each of the first pair of modes has a mode number of2 and each of the second pair of modes has a mode number of 3.

In one embodiment, the fine readout circuit is configured to drive thethird normal mode at a first drive frequency and to drive the fourthnormal mode at a second drive frequency, and wherein the fine readoutcircuit is configured to derive the measurement of the differencebetween the true angular rate of the CVG resonator and the coarsemeasurement by adjusting the first drive frequency and the second drivefrequency so that the difference between the first drive frequency andthe second drive frequency is proportional to the coarse measurement.

In one embodiment, the fine readout circuit is configured to derive themeasurement of the difference between the true angular rate of the CVGresonator and the coarse measurement by adjusting a natural frequency ofthe third normal mode and a natural frequency of the fourth normal modeso that the difference between the natural frequency of the third normalmode and the natural frequency of the fourth normal mode is proportionalto the coarse measurement.

In one embodiment, the fine readout circuit is configured to adjust thenatural frequency of the third normal mode by adjusting a bias voltageapplied to a tuning electrode coupled to the third normal mode.

According to an embodiment of the present invention there is provided amethod for operating an angular sensor including a coarse readoutcircuit configured with a coarse scale factor and a fine readout circuitconfigured with a fine scale factor, greater than the coarse scalefactor, the method including: generating a coarse measurement of anangular rate of the angular sensor using the coarse readout circuit, andmeasuring, with the fine readout circuit, the difference between thetrue angular rate of the angular sensor and the coarse measurement.

In one embodiment, the measuring, with the fine readout circuit, of thedifference between the true angular rate of the angular sensor and thecoarse measurement includes adjusting a drive frequency of a mode of aCoriolis vibratory gyroscope resonator of the angular sensor by anamount proportional to the coarse measurement.

In one embodiment, the angular sensor includes a mechanical tilt stagewith two rotational degrees of freedom, wherein: the CVG resonator issecured to the tilt stage, and the coarse readout circuit is configuredto generate a control signal proportional to the coarse measurement ofthe true angular rate of the CVG resonator and apply the control signalto the mechanical tilt stage to reduce the rotation of the tilt stage.

In one embodiment, the measuring, with the fine readout circuit, of thedifference between the true angular rate of the angular sensor and thecoarse measurement includes adjusting the rate of precession of a 2degree of freedom mechanical tilt stage by an amount proportional to thecoarse measurement.

According to an embodiment of the present invention there is provided asensor including: an acceleration or magnetic field sensitivemicroelectromechanical systems (MEMS) resonator, configured to oscillatein at least a first normal mode and a second normal mode, a coarsereadout circuit configured to: drive the first normal mode, measure amotion of the first normal mode, and derive from the measured motion acoarse measurement of the true acceleration or true external magneticfield; and a fine readout circuit configured to: drive the second normalmode, measure a motion of the second normal mode, and derive from themeasured motion and the coarse measurement a measurement of thedifference between the true acceleration or true external magnetic fieldand the coarse measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated and understood with reference to the specification, claims,and appended drawings wherein:

FIG. 1 is a block diagram of a high dynamic range gyroscope, accordingto an embodiment of the present invention;

FIG. 2 is a graph, including an enlarged view inset, of a coarse outputand a fine output of a high dynamic range gyroscope as a function ofangular rate, according to an embodiment of the present invention;

FIG. 3 is a mode pattern diagram of an n=2 mode and an n=3 mode,according to an embodiment of the present invention;

FIG. 4 is an illustration of a disk resonator, according to anembodiment of the present invention;

FIG. 5 is a block diagram of a high dynamic range gyroscope, accordingto an embodiment of the present invention;

FIG. 6 is a block diagram of a system for introducing an offset from acoarse readout circuit into a fine readout circuit, according to anembodiment of the present invention;

FIG. 7 is a block diagram of a system for introducing an offset from acoarse readout circuit into a fine readout circuit, according to anotherembodiment of the present invention;

FIG. 8 is a block diagram of a system for introducing an offset from acoarse readout circuit into a fine readout circuit, according to anotherembodiment of the present invention; and

FIG. 9 is a block diagram of a system for introducing an offset from acoarse readout circuit into a fine readout circuit, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of a highdynamic range gyroscope provided in accordance with the presentinvention and is not intended to represent the only forms in which thepresent invention may be constructed or utilized. The description setsforth the features of the present invention in connection with theillustrated embodiments. It is to be understood, however, that the sameor equivalent functions and structures may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention. As denoted elsewhere herein, like elementnumbers are intended to indicate like elements or features.

Referring to FIG. 1, in one embodiment a Coriolis vibratory gyroscope(CVG) includes a sensor head 110, a coarse readout circuit 115, a finereadout circuit 120 and a summing circuit 125. The sensor head includesa resonator, such as a disk resonator, one or more actuators forapplying force to the resonator, and one or more sensors for sensing thedisplacement or deformation of the resonator in response to appliedforces and in response to any rotation of the CVG. In one embodiment theactuators are pairs of parallel, closely spaced electrodes, each pairhaving the configuration of a parallel-plate capacitor. One electrode ofeach pair may be on the resonator, and the other electrode of the pairmay be secured, for example, to the housing of the CVG, so that when avoltage is applied across the electrodes, an electric field formsbetween the electrodes producing an attractive electric force betweenthem. The same or similar electrodes may also be used to detect thedisplacement of a portion of the resonator with respect to the housing.For example, if a DC voltage is applied to a pair of electrodes, thenthe change in capacitance resulting from a change in separation betweenthe electrodes may cause a current to flow onto one electrode of thepair and an equal current to flow away from the other electrode of thepair. Such a current may be amplified and converted to a voltage, forexample, with a transimpedance amplifier (TIA).

As described in further detail below, the resonator may oscillate invarious normal modes, and rotation of the resonator may cause energy tocouple from one normal mode into another. This coupling may be measuredand used to infer the angular rate of the CVG.

Referring to FIG. 2, in one embodiment, the coarse readout circuit 115senses the angular rate with a relatively small scale factor. Curve 210shows the output of the coarse readout circuit as a function of rotationrate. For example, the coarse readout circuit may generate a 19-bit datastream, each 19-bit number in the data stream representing theinstantaneous angular rate of the CVG, with a proportionality factor or“scale factor” equal to 2¹⁸ (the 19^(th) bit being a sign bit) dividedby the maximum measurable rate. For example, 2¹⁸ being 262,144, for aCVG capable of measuring a maximum angular rate of ±900 degrees persecond, the scale factor may be 262,144/900=291.3 bits per degrees persecond and the rate resolution is 900/262,144=0.0034 degrees per second.In some embodiments the coarse readout circuit includes 24-bitanalog-to-digital and digital-to-analog converters, which may haveinherent noise, so that the effective number of bits may be 19 or 20bits. FIG. 2 shows a lower resolution, 4-bit ADC for clarity, so thatthe 16 ADC output levels corresponding to the possible 4-bit numbers arediscernible.

The output of the coarse readout circuit 115 is fed to the fine readoutcircuit 120 as an offset or bias adjustment, so that the output range ofthe fine readout circuit 120 is significantly smaller, and a largerscale factor may be used. For example, if the difference between theangular rate and the digital representation of the angular rate at theoutput of the coarse readout circuit 115 is one-half bit of the coarsereadout circuit output (i.e., one half of 0.0034 degrees per second),then the residual angular rate measured by the fine readout circuit 120may be ±0.0034/2 degrees per second, i.e., ±0.0017 degrees per second.Accordingly, the scale factor of the fine readout circuit 120 may beselected to be significantly greater than the scale factor of the coarsereadout circuit 115. For example, if the fine readout circuit 120 alsogenerates a 19-bit data stream, each 19-bit number in the data streamrepresenting the residual instantaneous angular rate of the CVG (aftersubtraction of the coarse readout circuit offset), then the resolutionof the fine readout circuit 120 may be 0.0017 degrees per second dividedby 2¹⁸ (the 19th bit being a sign bit), i.e., 6.5×10⁻⁹ degrees persecond, or 24 microdegrees per hour. In FIG. 2, curve 220 shows theoutput of the fine readout circuit without offset subtraction, and curve225 shows the output of the fine readout circuit with offsetsubtraction.

In one embodiment, a resonator configured to oscillate in two pairs ofmodes may be used to implement coarse and fine angular rate sensingusing a single sensor head. Referring to FIG. 3, such a resonator may bea disk or other cylindrically symmetric structure and the modes may bemodes (referred to as “wineglass modes”) having 2-fold or 3-foldrotational symmetry as shown. In other embodiments a non-cylindricallysymmetric CVG resonator (e.g., a lumped mass octagon resonator with afirst pair of modes in the X and Y directions and a second pair of modesrotated 45 degrees relative to the first) may be used. The modes with2-fold rotational symmetry may have a mode number of 2 and mayaccordingly be referred to as n=2 modes, and the modes with 3-foldrotational symmetry may have a mode number of 3 and may accordingly bereferred to as n=3 modes. Each mode number may correspond to a family ofnominally degenerate modes, i.e., modes that would be perfectlydegenerate (i.e., that would have the same natural frequency) if theresonator shape and boundary conditions had perfect cylindricalsymmetry. FIG. 3 shows an undeformed resonator 310, along with the shape320 that it may take when deformed in the shape of an n=2 mode and theshape 330 that it may take when deformed in the shape of an n=3 mode.

The coarse readout circuit 115 may use a pair of degenerate modes (e.g.a pair of n=2 modes) to generate the coarse angular rate signal. Forexample, referring to FIG. 4, the resonator may have 16 electrodesincluding a first drive electrode 410, a second drive electrode 415, afirst sense electrode 420, and a second sense electrode 425. A first n=2mode having a first mode shape 430 may be driven by the first driveelectrode 410 and sensed by the first sense electrode 420, and a secondn=2 mode having a second mode shape 435 may be driven by the seconddrive electrode 415 and sensed by the second electrode 425. In otherembodiments, the resonator may have a different number of electrodes,e.g., 24 electrodes. In some embodiments more than one electrode may bedriven simultaneously or concurrently to drive a particular mode, and/ormore than one electrode may be sensed simultaneously or concurrently tosense displacement in a particular mode.

Referring to FIG. 5, in one embodiment the resonator may be outfittedwith a first sense electrode 420 and a first drive electrode 410,coupled to the first mode of the resonator. A first control loop forcontrolling the oscillation of the first mode may include a firsttransimpedance amplifier (TIA) 505 connected to the first senseelectrode 420, a first analog to digital converter 510, a firstautomatic gain control block 515, a first phase locked loop block 520, afirst digital to analog converter 525, and a first driver 530. The firsttransimpedance amplifier 505, the first analog to digital converter 510,the first digital to analog converter 525, and the first driver 530 maybe part of a digital to analog conversion and analog signal conditioningcircuit 550. The first automatic gain control block 515 and the firstphase locked loop block 520 may be part of a digital control andsynthesis circuit 552, which may be an entirely digital circuitimplemented, for example, in a field-programmable gate array (FPGA). Thefrequency reference 553 may include a first stable frequency reference535 (such as a rubidium atomic clock or a chip-scale atomic clock)operating at a relatively high internal frequency, e.g., 10 MHz. Afrequency divider 540 may divide the frequency down to a frequency, atthe output of the frequency reference 553, that is near the naturalfrequencies of the first and second normal modes, e.g., 20 kHz. Thefrequency reference 553 may also include a reference voltage generator545 that takes the stable frequency provided by the frequency reference535 and converts it into a stable reference voltage. In operation, thefirst mode may, in response to drive signals applied by the firstcontrol loop, oscillate at the frequency reference output frequency,e.g., 20 kHz. In one embodiment, the first TIA 505 generates a signalproportional to the displacement of the disk in the first mode, and thefirst analog to digital converter 510 generates from it a digital datastream representing the displacement of the disk in the first mode. Thefirst automatic gain control block 515 measures the amplitude of themotion, compares it to a target (or “setpoint”) value (e.g., apre-programmed operating amplitude), generates an amplitude error signalproportional to the difference between the amplitude of the motion andthe target, and generates an amplitude correction signal from theamplitude error signal. The amplitude correction signal may begenerated, for example, by processing the amplitude error signal with aproportional-integral-differential (PID) controller. In someembodiments, the first stable frequency reference 535 is shared bymultiple sensor systems, e.g., it is shared by three gyroscopes that arecombined to form a three-axis gyroscope.

In other embodiments, the system may operate without a high-stabilityexternal frequency reference (which may result in a loss of performanceand a reduction in cost), or without any external frequency reference.In the latter case one of the CVG modes may be used as the resonator foran oscillator; for example, the signal from a sense electrode sensingthe first mode or the second mode may be fed back to a drive electrodedriving the same mode, so that the loop gain exceeds one in a frequencyinterval around the natural frequency of the mode and with a phase thatresults in instability (i.e., oscillation at the frequency of aright-half-plane pole created by the feedback). A point in this loop(e.g., the output of a TIA or the output of a driver) may then be used(instead of the output of the frequency reference 553) as the source ofthe frequency reference signal used for both modes.

In a parallel path, the output from the first analog to digitalconverter 510 may also be processed by the first phase locked loop block520 to measure the phase error of the displacement of the CVG in thefirst mode, and to generate a corresponding correction signal andsinusoidal drive signal. For example, the first phase locked loop block520 may control the phase of the displacement of the disk in the firstmode to be substantially in phase with the frequency reference outputsignal as follows. The first phase locked loop block 520 may generate aphase-shifted signal 90 degrees out of phase with frequency referenceoutput signal, multiply this phase-shifted signal with the measureddisplacement in the first mode, and process the result with a low-passfilter, e.g., the loop filter in the PLL. The DC component of theproduct of the measured displacement and the phase-shifted sinusoidalsignal will be zero if the measured displacement in the first mode isperfectly in phase with the frequency reference output signal, and itwill be non-zero and proportional to the phase error (for a small phaseerror) when the measured displacement in the first mode is not perfectlyin phase with the frequency reference output signal. In otherembodiments, the relative phase of the measured displacement in thefirst mode and the frequency reference output signal may be measuredusing other methods, e.g., by fitting each with a linear combination ofa sine function and a cosine function. A phase error may then becalculated by taking the difference between (i) the phase differencebetween the measured displacement in the first mode and the frequencyreference output signal and (ii) a first phase target (i.e., a targetphase difference or “phase setpoint”). Once a phase error has beencalculated, a phase correction signal may be generated from the phaseerror by processing the phase error with a PID controller, either in thephase locked loop block 520 or in the first automatic gain control block515.

The automatic gain control block 515 may then generate a digital drivesignal, that tends to reduce the amplitude error and the phase error,from the amplitude correction signal, the phase correction signal, thephase-shifted signal and the frequency reference output signal. Thisdrive signal may have an amplitude proportional to the amplitudecorrection signal (or proportional to the amplitude correction signalplus a constant offset) and a phase set by the phase correction signal.For example, when the phase error is zero, the phase of the drive signalmay be zero (i.e., it may produce a force that leads the displacement by90 degrees), and when the phase error is not zero, the phase of thedrive signal may be proportional to the phase correction signal.Similarly, the amplitude of the drive signal, when the amplitude erroris zero, may be just sufficient to counteract mechanical loss in theresonator, so that the amplitude of the motion in the first mode remainsconstant. When the amplitude correction signal is positive, theamplitude of the drive signal may be greater, and when the amplitudecorrection signal is negative, the amplitude of the drive signal may besmaller. The drive signal may be applied to the first drive electrode410, to produce a corresponding force on the resonator.

A second control loop for controlling the oscillation of the second modemay similarly include a second transimpedance amplifier 555 (TIA), asecond analog to digital converter 560, a second automatic gain controlblock 565, a second phase locked loop block 570, a second digital toanalog converter 575, and a second driver 580. The amplitude and phaseof the displacement in the second mode may be controlled in an analogousmanner to have a particular amplitude and a particular phase relative tothe output of the frequency reference 553. In one embodiment the secondmode is controlled to oscillate with a phase that is 90 degreesdifferent from that of the first mode (i.e., the oscillation of thesecond mode is locked to a second phase target that differs by 90degrees from the first phase target) and with an amplitude that is about10% of that of the first mode. When the CVG rotates, energy couples fromthe first mode into the second mode with a phase that is 90 degrees outof phase with the nominal oscillation of the second mode, resulting in aphase error in the second mode that is then suppressed by the secondcontrol loop. The rate of rotation (if any) may then be inferred fromthe phase error signal or by the phase correction signal in the secondloop.

The angular rate output from the coarse readout circuit 115 may, asmentioned above, be a digital data stream, with each number in the datastream representing the instantaneous angular rate of the CVG. Any ofseveral configurations may be employed to feed this data stream into thefine readout circuit 120, to enable the fine readout circuit 120 tooperate at higher resolution. Referring to FIG. 6, in one embodiment, inthe fine readout circuit (e.g., the n=3 readout circuit) the output ofthe second transimpedance amplifier 555 may be connected both to thesecond analog to digital converter 560 and to a summing circuit 605. Theoutput of the coarse readout circuit 115 may be connected to coarsesignal input 610, modified by modulator block 615, converted to ananalog signal by an analog to digital converter 617, and fed into thesumming circuit 605. The modulator block may be configured to generate,at its output, a waveform that is the opposite of the waveform expectedat the output of the second transimpedance amplifier 555 when the CVG isrotating at the rate indicated by the coarse signal. In this manner,when the output of the modulator block 615 and the output of the secondtransimpedance amplifier 555 are summed in the summing circuit 605, theoutput of the summing circuit 605 is small, representing only theresidual of the rotation measured in the fine readout circuit 120 afterthe signal from the coarse readout circuit 115 has been subtracted. Theoutput of the summing circuit 605 is then fed to a third analog todigital converter 620, and to a fine signal processing block 625. Thegain or scale factor of the third analog to digital converter 620 may beselected to be relatively large, so that the relatively small signal atthe output of the summing circuit 605 may span the entire range, ornearly the entire range, of the third analog to digital converter 620,and, accordingly, the resolution of the third analog to digitalconverter 620 may be relatively high compared to the resolution of thesecond analog to digital converter of the coarse readout circuit. Thefine signal processing block 625 may then infer the fine component ofthe angular rate from the output of the third analog to digitalconverter 620.

Referring to FIG. 7, in another embodiment, similar processing may beperformed entirely in the digital domain as shown. In this embodiment,the digital output signal from the coarse readout circuit 115 is fedinto the digital control and synthesis circuit 552, within which it issent to the second automatic gain control block 565 and to the secondphase locked loop block 570 (not shown in FIG. 7), and also to a summingcircuit 605, which in the embodiment of FIG. 7 is a digital summingcircuit. As in the embodiment of FIG. 6, the modulator block 615 isconfigured to generate, at its output, a waveform that is the oppositeof the waveform expected at the output of the second transimpedanceamplifier 555 when the CVG is rotating at the rate indicated by thecoarse signal. In this manner, when the output of the modulator block615 and the output of the second transimpedance amplifier 555 are summedin the summing circuit 605, the output of the summing circuit 605 issmall, representing only the residual of the rotation measured in thefine readout circuit 120 after the signal from the coarse readoutcircuit 115 has been subtracted.

Referring to FIG. 8, in a third embodiment, the signal from the coarsereadout circuit 115 is used to generate an offsetting bias in the finereadout circuit 120 by causing the first and second modes of the pair ofmodes used by the fine readout circuit 120 to oscillate at differentfrequencies. This frequency difference may result in a bias, in the finereadout circuit 120, proportional to the frequency difference.

The frequency difference may be produced, for example, by splitting thesignal from the first stable frequency reference 535 to two dividers805, 810 that generate different reference frequency signals for thefirst and second modes of the fine readout circuit 120. In oneembodiment a frequency setting circuit 815 receives the output signal ofthe coarse readout circuit 115 and calculates from it a selected pair offrequencies that, when provided to the control circuits for the firstand second modes of the fine readout circuit 120, results in a bias thatsubstantially cancels the effect of the rotation of the CVG in the finereadout circuit 120. The dividers 805, 810 may then be programmed (e.g.,also by the frequency setting circuit 815, or by circuits in therespective dividers) with divider ratios that cause the dividers 805,810 to produce, from the internal frequency, the selected frequencies.

In the embodiment of FIG. 8, the effect of introducing a bias throughthe adjustment of the respective drive frequencies of the two modes ofthe fine readout circuit 120 is that the output of the fine readoutcircuit 120 is confined to a substantially smaller range than the rangeof outputs that would be present in the absence of the offset providedby the coarse readout circuit 115; the output range of the fine readoutcircuit 120 may be, for example, ±0.0017 degrees per second, instead of±900 degrees per second. This may allow the fine stage output to bemeasured with correspondingly finer resolution.

Referring to FIG. 9, in one embodiment a bias may be introduced in thefine readout circuit 120 by adjusting the natural frequencies of thepair of modes coupled to the fine readout circuit 120. The naturalfrequency of any of the normal modes may be adjusted by applying a DC,or slowly varying, potential to a tuning electrode coupled to the normalmode. The tuning electrode may be a drive electrode, a sense electrode,or another electrode coupled to the normal mode. The attractive forcedue to a potential difference across the two electrodes may vary withthe separation of the electrodes, e.g., due to fringing fields at theedges of the electrodes and because, if the potential difference isconstant, the change in capacitance with separation may result in achange in the charge on the electrodes. This variation in force withdistance may have the effect of an additional (negative) spring forcesuperimposed on the mechanical internal restoring force of theresonator, so that a DC voltage applied to any pair of electrodes mayaffect (e.g., lower) the natural frequency of the first and/or secondnormal mode.

This characteristic may be used to adjust, based on the output of thecoarse readout circuit 115, the natural frequency of one or both modescoupled to the fine readout circuit 120 so as to introduce a bias intothe fine readout circuit 120 canceling the rotational rate measured bythe coarse readout circuit 115. In one embodiment a frequency settingcircuit 915 receives the output signal of the coarse readout circuit 115and calculates from it tuning voltages that, when applied to the tuningelectrodes for the first and second modes of the fine readout circuit120, result in a bias that substantially cancels the effect of therotation of the CVG in the fine readout circuit 120. As in theembodiment of FIG. 8, this cancellation may allow the fine stage outputto be measured with significantly finer resolution.

In some embodiments the bias cancellation from the coarse readout isintroduced to the fine readout through a 2DOF (or more) mechanical tiltstage controlled by the coarse readout circuit to cause the input axisof the gyro to precess in a manner opposing the coarse rotation signal.In some embodiments, a closed loop system is utilized, in which a gyrorate output is nulled and the signal strength necessary to do so is ameasure of the gyro rotation rate.

The techniques of embodiments of the present invention may be applied toother sensors such as accelerometers and magnetometers, e.g., combininga coarse sensor and a fine sensor, with different scale factors, andwith the coarse sensor providing an offset to the fine sensor.

It will be understood that, although the terms “first”, “second”,“third”, etc., may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, a first element, component, region, layer or sectiondiscussed below could be termed a second element, component, region,layer or section, without departing from the spirit and scope of theinventive concept.

Spatially relative terms, such as “beneath”, “below”, “lower”, “under”,“above”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that such spatially relative terms are intended to encompassdifferent orientations of the device in use or in operation, in additionto the orientation depicted in the figures. For example, if the devicein the figures is turned over, elements described as “below” or“beneath” or “under” other elements or features would then be oriented“above” the other elements or features. Thus, the example terms “below”and “under” can encompass both an orientation of above and below. Thedevice may be otherwise oriented (e.g., rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein shouldbe interpreted accordingly. In addition, it will also be understood thatwhen a layer is referred to as being “between” two layers, it can be theonly layer between the two layers, or one or more intervening layers mayalso be present.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the inventiveconcept. As used herein, the terms “substantially,” “about,” and similarterms are used as terms of approximation and not as terms of degree, andare intended to account for the inherent deviations in measured orcalculated values that would be recognized by those of ordinary skill inthe art. As used herein, the term “major component” means a componentconstituting at least half, by weight, of a composition, and the term“major portion”, when applied to a plurality of items, means at leasthalf of the items.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. Expressions such as “at least one of,” when preceding alist of elements, modify the entire list of elements and do not modifythe individual elements of the list. Further, the use of “may” whendescribing embodiments of the inventive concept refers to “one or moreembodiments of the present invention”. Also, the term “exemplary” isintended to refer to an example or illustration. As used herein, theterms “use,” “using,” and “used” may be considered synonymous with theterms “utilize,” “utilizing,” and “utilized,” respectively.

It will be understood that when an element or layer is referred to asbeing “on”, “connected to”, “coupled to”, or “adjacent to” anotherelement or layer, it may be directly on, connected to, coupled to, oradjacent to the other element or layer, or one or more interveningelements or layers may be present. In contrast, when an element or layeris referred to as being “directly on”, “directly connected to”,“directly coupled to”, or “immediately adjacent to” another element orlayer, there are no intervening elements or layers present.

Any numerical range recited herein is intended to include all sub-rangesof the same numerical precision subsumed within the recited range. Forexample, a range of “1.0 to 10.0” is intended to include all subrangesbetween (and including) the recited minimum value of 1.0 and the recitedmaximum value of 10.0, that is, having a minimum value equal to orgreater than 1.0 and a maximum value equal to or less than 10.0, suchas, for example, 2.4 to 7.6. Any maximum numerical limitation recitedherein is intended to include all lower numerical limitations subsumedtherein and any minimum numerical limitation recited in thisspecification is intended to include all higher numerical limitationssubsumed therein.

Any digital circuit employed in embodiments of the present invention,such as the digital control and synthesis circuit 552, may beimplemented with a processor. The term “processor” is used herein toinclude any combination of hardware, firmware, and software, employed toprocess data or digital signals. Processing unit hardware may include,for example, application specific integrated circuits (ASICs), generalpurpose or special purpose central processing units (CPUs), digitalsignal processors (DSPs), graphics processing units (GPUs), andprogrammable logic devices such as field programmable gate arrays(FPGAs).

Although exemplary embodiments of a high dynamic range gyroscope havebeen specifically described and illustrated herein, many modificationsand variations will be apparent to those skilled in the art. Forexample, in some modes the coarse readout circuit 115 and the finereadout circuit 120 are connected to modes other than n=2 and n=3 modesrespectively, e.g., they may be connected to n=3 and n=2 modes,respectively. Accordingly, it is to be understood that a high dynamicrange gyroscope constructed according to principles of this inventionmay be embodied other than as specifically described herein. Theinvention is also defined in the following claims, and equivalentsthereof.

What is claimed is:
 1. A method for operating an angular sensorcomprising: a coarse readout circuit configured with a coarse scalefactor, and a fine readout circuit configured with a fine scale factor,greater than the coarse scale factor, the method comprising: generatinga coarse measurement of a true angular rate of the angular sensor usingthe coarse readout circuit, and measuring, with the fine readoutcircuit, a difference between the true angular rate of the angularsensor and the coarse measurement.
 2. The method of claim 1, wherein themeasuring, with the fine readout circuit, of the difference between thetrue angular rate of the angular sensor and the coarse measurementcomprises adjusting a drive frequency of a mode of a Coriolis vibratorygyroscope resonator of the angular sensor by an amount proportional tothe coarse measurement.
 3. The method of claim 1, wherein the measuring,with the fine readout circuit, of the difference between the trueangular rate of the angular sensor and the coarse measurement comprisesadjusting a rate of precession of a 2 degree of freedom mechanical tiltstage by an amount proportional to the coarse measurement.